Abstract

The quantum principles of superposition and entanglement have led to a recasting of the foundations of information and computation theory, and are especially helpful in understanding the nature of privacy. The most private information, exemplified by a quantum eraser experiment, is best regarded as existing only conditionally and temporarily—after the experiment is over no trace remains. Less private is classically‐secret information—quantum information that has decohered, and thus become recoverable in principle, though not in practice, from portions of the environment. Finally there is public information, which has been replicated so thoroughly throughout the environment as to be infeasible to conceal. The Internet has caused an explosion of public information, with the beneficial side effect of making it harder for despots to rewrite the history of their misdeeds, and it is tempting to hope that all macroscopic information is permanent, making such cover‐ups impossible in principle if not in practice. However, by comparing entropy flows into and out of the Earth with estimates of the planet’s storage capacity, we conclude that most macroscopic information—for example the pattern of sand grains on an ancient beach—is impermanent, in the sense of becoming irrecoverable in principle from the Earth though still recorded in the Universe.